How to bake textures with compositing applied?
You could try saving the image as close to the size of your texture as you can get and projection-mapping it (this does mean you'll need to render a few times, though). EDIT: You'd need to render at least one more time to get the back, and possibly a few more for more obscure angles and stitch the images together either via UV layers or externally, or, my favorite, with masking maps.